Cilia are motile cell organelles important in biomotility, sensory transduction and epithelial function in many organisms, including man. This study provides a comprehensive attack on the structure and function of, mainly, motile cilia, centering arond ultrastructural correlates of ciliary movement. The sliding microtubule hypothesis of ciliary motility developed in this laboratory will be extended, primarily by continued morphological analysis of beat stages in model cilia so that the structural basis of bend formation and bend propagation within the ciliary axoneme can be defined or delineated. Freeze-fracture of cilia has revealed new structures at the base of the organelle including the ciliary necklace and the related membrane-microtubule complex, whose function in ciliary physiology is presently unknown and will be sought. In ciliated epithelia, cellular control mechanisms can produce ciliary arrest. In some cases, the arrest can be induced by localized laser irradiation and it will spread systematically from cell to cell. The ionic and pharmacological conditions that produce arrest will be clarified. The functional status of epithelial cell junctions in cell coupling will be correlated with the intercellular passage of arrest. The intracellular mechanisms by which arrest is brought about will be investigated. It is likely that all motile cilia possess reasonably common mechanisms of motility and motility control, knowledge of which may enable us to understand or prevent ciliary malfunction that can precipitate, for example, respiratory disease.